Modern casinos are increasingly moving toward electronic and computerized implementations for their gaming machines. For example, slot machines historically were mechanical devices whose physical reels could be spun by pulling a lever on the side of the machine. Each symbol on each reel occupied a physical stop having the same probability of occurrence as all other stops on the reel, and the machine would pay out based on the combination of symbols appearing in a line across the reels (the “payline”) when all of the reels stopped spinning. Today, however, mechanical reels in slot machines are typically controlled electronically, such that different payline probabilities can be assigned to different symbols on the reels. The reels can be spun by pushing a button that activates the electronic control, although some machines may retain the traditional lever for entertainment value. In newer video slot machines, the physical reels are replaced by virtual reels whose symbols are displayed on a video screen, controlled by one or more computer processors. Some video slot machines have physical buttons for the player to press, while others are operated via touchscreen.
An electronic gaming machine is typically programmed, via software or firmware, to pay out as winnings, in the long run, a particular percentage of the money that is paid in by players as wagers. This is typically done by setting the probabilities for individual payouts in the machine's control system. For example, if a machine gives a payout of 10 times the player's wager with a probability of 5%, and a payout of 20 times the player's wager with a probability of 2%, then the machine has a theoretical payout percentage of 90%. The remaining 10% of the wagers are kept by the “house” (i.e., the slot machine operator—typically the casino) as profits. The payouts that result from various combinations of symbols appearing on a payline when the reels stop spinning are typically listed in a pay table that may be displayed somewhere on the machine. The probability of each payout can thus be controlled by setting the probabilities of the individual symbols that must co-occur on the payline to produce that payout.